In recent years, fuel cells have received much attention as next generation energy sources. In particular, polymer electrolyte fuel cells (PEFCs) in which a polymer membrane having proton conductivity is used as an electrolyte have high energy density, and are expected to find a wide range of applications, such as home cogeneration systems, power sources for mobile devices, and power sources for automobiles. An electrolyte membrane for a PEFC is required to serve not only as an electrolyte for conducting protons between a fuel electrode and an oxidant electrode but also as a partition for separating a fuel supplied to the fuel electrode and an oxidant supplied to the oxidant electrode. If either of these functions as an electrolyte and a partition is insufficient, the power generation efficiency of the fuel cell decreases. For this reason, polymer electrolyte membranes are required to have high proton conductivity, electrochemical stability and mechanical strength, and low permeability to fuels and oxidants.
Currently, perfluorocarbon sulfonic acid that has sulfonic acid groups as proton-conductive groups (for example, “Nafion (registered trademark)” manufactured by DuPont) is widely used for electrolyte membranes for PEFCs. Membranes made of perfluorocarbon sulfonic acid have high electrochemical stability. However, these membranes are very expensive because fluororesin materials for them are not general purpose materials and their synthesis processes are complicated. The high cost of electrolyte membranes is a major obstacle to the practical use of PEFCs. Direct methanol fuel cells (DMFCs) are a type of PEFCs in which a solution containing methanol is supplied to a fuel electrode. DMFCs are advantageous in ease of supply of fuels and portability, and their potential practical applications are attracting attention. However, perfluorocarbon sulfonic acid membranes are highly permeable to methanol, which makes them difficult to use in DMFCs.
As an alternative to such perfluorocarbon sulfonic acid membranes, hydrocarbon-based polymer electrolyte membranes are being developed. Resin materials for hydrocarbon-based electrolyte membranes are less expensive than fluororesin materials, so the use of these electrolyte membranes is expected to reduce the cost of PEFCs.
JP 2000-510511 T discloses, as a hydrocarbon-based polymer electrolyte membrane, an electrolyte membrane containing a polyimide resin formed by polycondensation of a tetracarboxylic dianhydride, an aromatic diamine having a proton-conductive group, and another aromatic diamine having no proton-conductive group. This publication describes that this electrolyte membrane has high mechanical and electrochemical stability and can be produced at lower cost than perfluorocarbon sulfonic acid membranes. However, the technique disclosed in JP 2000-510511 T gives no consideration to the methanol permeation resistance property (i.e., methanol blocking property) of electrolyte membranes. The electrolyte membrane disclosed in this publication does not necessarily have high resistance to methanol permeation.
JP 2003-68326 A discloses a similar polyimide-based polymer electrolyte membrane. In this publication, an attempt is made to overcome a disadvantage of readily hydrolyzable imide bonds to form a polyimide-based electrolyte membrane having high resistance to hydrolysis (long-term water resistance). However, the technique of JP 2003-68326 A also gives no consideration to the methanol permeation resistance property of electrolyte membranes, and the electrolyte membrane disclosed in this publication does not necessarily have high resistance to methanol permeation.